1. Field of the Invention
The present invention relates to the stimulating of subterranean formations surrounding oil wells, gas wells, water wells, injection wells and similar bore holes.
2. Brief Description of the Prior Art
The flow of oil from a subterranean formation to a well bore depends, among other factors, upon the permeability of the formation. Often permeability is not sufficient to allow a desired flow rate of fluids, such as crude oil or natural gas, from the formation. In such a case the formation can be treated to increase its permeability.
Hydraulic fracturing is one type of treatment used to increase the production of fluids. Accordingly many methods have been developed which are useful for hydraulically fracturing a subterranean formation penetrated by a well bore. Commonly, in the art of hydraulic fracturing, a fluid is introduced into the formation sought to be fractured by a conduit, such as tubing or casing, disposed in a well bore. The fluid is introduced at a rate and pressure sufficient to produce a fracture in the formation and to extend the produced fracture from the well bore into the formation. The fluid can include a propping agent, for example sand, which results in placement of the propping agent within the fracture thus produced. Following the fracturing treatment, the introduced fluid is recovered from the formation but the proppant remains in the produced fracture to prevent complete closure of the formation. Thus, a highly conductive channel extending from the well bore into the formation is created through which formation fluids can easily flow.
Conventional fracturing fluids containing water soluble polymers, such as high molecular weight polysaccharides, are well known in the art. Polysaccharides (e.g. galactomannan derivatives and cellulose derivatives) are readily solubilized in aqueous liquids (used herein to mean any fluid containing some water) and some polysaccharides can develop enhanced viscosity in the presence of mono- and other poly-valent cations. These polymer solutions are typically crosslinked with various metal ions, such as titanium or zirconium, to form viscoelastic gels. The term "gel" is used hereafter to mean any crosslinked polymer solution. Fracturing fluids based on polysaccharide-metal ion chemistry are limited in application because the polysaccharide backbone undergoes hydrolysis and oxidation at elevated temperatures. As a result, the viscosity and gel structure are rapidly lost when the fluid is exposed to high formation temperatures, used hereafter to mean temperatures in the range of 350.degree. F. and above.
Many low permeability formations that require hydraulic fracturing to increase oil or gas production have temperatures in excess of 350.degree. F. In order to effectively fracture this type of formation, the fracturing fluid must be temperature-stable, i.e., maintain viscosity and gel structure for a sufficient amount of time at formation temperature so that the fracture extends and the propping agent is placed at the desired distance from the well bore.
Synthetic polymers that can produce and maintain viscosity at temperatures above 350.degree. F. have been developed for well stimulation, for example fracture acidizing. However, other deficiencies have prevented synthetic polymers from being used to any great extent for fracturing high temperature formations. Non-ionic synthetic polymers, such as polyacrylamide, have limited crosslinkability and therefore cannot attain sufficient viscosity and structure for fracturing and/or transporting proppant. Additional crosslink sites, for example carboxylate groups, may be incorporated into the polymer, but carboxylate groups induce salt sensitivity of the polymer. A polymer for hydraulic fracturing must be salt tolerant because the aqueous media is normally a brine which is needed to reduce or prevent swelling of the formation clays.
The present invention overcomes the above described deficiencies of polysaccharides and synthetic polymers used in conventional water-based fracturing fluids. The present invention provides a method for effectively stimulating high temperature subterranean formations utilizing a novel crosslinkable polyampholyte.